the emigration of Muhammad and his followers (muhajirun) from Mecca to Yathrib (Medina), which Muslim tradition dates at Sept. 8–21 (or 24), 622.
During the reign of the caliph Umar I (634–44), the year of the Hegira was proclaimed the first year in the Muslim era; hence, the Muslim era is also called the Hegira. Calendars of the lunar Hegira are used in many countries of the Near and Middle East and in certain other countries; calendars of the solar Hegira are used in Iran, Afghanistan, Turkey, and elsewhere. When converted to the Gregorian system, the beginning of the lunar Hegira corresponds to July 16, 622, and that of the solar Hegira to Mar. 20–21, 622, the vernal equinox.
The Muslim lunar year contains 354 days, and the distribution of days by month in the Hegira calendars differs from the distribution in the Gregorian calendar (used virtually throughout the world), so that the translation of dates from the Hegira to the Gregorian system and vice versa requires special formulas and tables. For example, the year 1976 of the Gregorian calendar corresponds to 1396/1397 of the lunar Hegira and to 1355/1356 of the solar Hegira.